1. Field of the Invention
The present invention relates to an optical pick-up apparatus comprising a semiconductor laser emitting a laser beam, a converging optical system for projecting the laser beam emitted by the semiconductor laser onto an optical record medium as a fine spot, a hologram means for diffracting the laser beam reflected by said optical record medium to produce at least +1-order and -1-order beams, and a photodetector means for receiving said +1-order and -1-order beams diffracted by said hologram means to produce a tracking error signal.
2. Related Art Statements
An optical pick-up apparatus of the kind mentioned above has been described in Japanese Patent Application Laid-open Publication Kokai Hei 4-248134 published on Sep. 3, 1992. Further, the inventor of the instant application has proposed, in a Japanese Patent Application Laid-open Publication Kokai Hei 5-307759 published on Nov. 19, 1993 which is later than a priority date of the present application, an optical pick-up apparatus, in which a single main beam and two sub-beams are projected onto an optical record medium, a focusing error is detected from the main beam reflected by the optical record medium and a tracking error is detected from the two sub-beams. Such a tracking error detecting system has been usually called a three beam method.
FIGS. 1 to 4 illustrate the optical pick-up apparatus described in the above Laid-open Publication Kokai Hei 5-307759. As shown in FIG. 1, a semiconductor laser, photodetector and hologram are formed as a single unit 1. A laser beam emanating from the unit 1 is made incident upon an optical record medium 4 such as optomagnetic record medium or phase-change type record medium by means of a stop 2 and an objective lens 3. The laser beam reflected by the record medium 4, i.e. a return laser beam is made incident upon the unit 1 by means of the objective lens 3 and stop 2.
FIG. 2 shows a detailed construction of the unit 1. The unit comprises a base plate 5 on which a semiconductor substrate 6 is provided. Above the base plate 5 there is arranged a hologram optical element (HOE) 8 via a spacer 7. On the semiconductor substrate 6, there are mounted a semiconductor laser 9 and photodetectors 11, 12, 13 and 14, 15, 16 as depicted in FIG. 3. The three photodetectors 11, 12 and 13 are arranged on one side of the semiconductor laser 9 viewed in a direction x, and three photodetectors 14, 15 and 16 are arranged on the other side of the semiconductor laser 9. The photodetectors 11, 12 and 13 are aligned in a direction y perpendicular to the direction x, and similarly, the photodetectors 14, 15 and 16 are aligned in the direction y. As shown in FIG. 4, the semiconductor laser 9 is arranged on a bottom of a depression 6a formed in the surface of the semiconductor substrate 6 by etching, and the laser beam emitted by the semiconductor laser is reflected by an inclined side wall 6b of the depression. That is to say, the laser beam emitted by the semiconductor laser 9 in a direction parallel to the surface of the semiconductor substrate 6 is reflected by the side wall 6b in a direction perpendicular to the surface of the semiconductor substrate.
On a surface of the HOE 8 facing with the semiconductor substrate 6 there are provided gratings 8a which divide the laser beam emitted by the semiconductor laser 9 into three beams, i.e. 0-order beam, +1-order beam and -1-order beam. On an opposite surface of the HOE 8 there is formed a hologram pattern 8b which gives the +1-order beam and -1-order beam opposite powers.
The laser beam emitted by the semiconductor laser 9 is reflected by the mirror surface 6b and then is diffracted by the gratings 8a into the 0-order beam, +1-order beam and -1-order beam. The 0-order beam is utilized as a main beam and +1-order and -1-order beam are used as sub-beams. These three beams are made incident upon the optical record medium 4 such that the 0-order main beam is projected onto a center of an information track on the optical record medium, the +1-order beam is made incident upon one of edges of said track and the -1-order beam is made incident upon the other edge of said track. The three beams are reflected by the optical record medium and are made incident upon the hologram pattern 8b. Then, each of the three beams is diffracted by the hologram pattern 8b, and +1-order and -1-order beams of the main beam are made incident upon the photodetectors 12 and 15, +1-order and -1-order beams of one of the two sub-beams are made incident upon the photodetectors 11 and 14 and +1-order and -1-order beams of the other sub-beam are made incident upon the photodetectors 13 and 16.
In the optical pick-up apparatus shown in FIGS. 1 to 4, the hologram pattern 8b is constructed to give the .+-.1-order beams opposite powers to each other, so that it is possible to detect the focusing error from output signals of the photodetectors 12 and 15 by the beam size method. The tracking error is detected from output signals of the photodetectors 14 and 16 by the three beam method.
The above mentioned optical pick-up apparatus can be small in size and a reliability can be improved owing to the fact that the semiconductor laser 9, photodetectors 11-16 and hologram optical element 8 are formed as the single unit 1. However, various experiments and studies have revealed that the above mentioned optical pick-up apparatus has the following drawbacks. When a phase-change type record medium is used as the optical record medium, a ratio of intensity of the two sub-beams to be used for deriving the tracking error is not constant due to a fact that the reflectance of the phase-change type record medium changes locally or partly, so that it is impossible to obtain a stable tracking error signal. Furthermore, when use is made of a magneto-optical record medium as the optical record medium, a groove formed in the magneto-optical record medium has generally a depth of .lambda./8 (.lambda. is a wavelength of the laser beam), and thus it is impossible to attain a large tracking error signal by the three beam method.